Project: Innovative Waterfront Retaining Wall Saves Condo

Where: Branson, Missouri

Overview

Record precipitation in 2008 prompted the US Army Corps of Engineers (USACE) to open Table Rock Dam’s flood gates near Branson, Missouri. The subsequent flow into Lake Taneycomo below the dam was the highest on record since the dam’s construction and resulted in significant scour along the lake’s banks. The slope below one condominium fell away, leaving the building hanging without support over a drop of roughly 30 feet to the water. GeoEngineers provided geotechnical exploration and instrumentation, innovative engineering design and construction observation to help save and protect the condominiums at risk.

Approach

• GeoEngineers conducted a geotechnical investigation of the project site, including installing inclinometers to continue monitoring the eroded slope.
• The initial plan was to recover and protect the banks with a traditional riprap barrier (stone or gravel that protects against erosion). However, the USACE rejected this approach because the riprap would need to extend too far into Lake Taneycomo to be effective, potentially changing the river hydraulics and disturbing the important fish habitat at the site.
• With traditional approaches unavailable, GeoEngineers focused on a retaining structure system that used conventional soil nails combined with vertical elements in a bottom-up sequence rather than traditional top-to-bottom excavation.
• GeoEngineers used finite element modeling to predict wall performance and optimize the design.
• GeoEngineers’ innovative design also called for constructing a riprap toe at the bottom of the wall to provide scour protection. The design added enough riprap in the slope to account for the amount of riprap that would slide─or launch─into scour holes created during flood events. This approach, otherwise known as a launchable stone toe, made it possible to build the wall with almost no excavation below the lake surface and had the added benefit of providing a platform for construction operations.

Results

• The hybrid soil nail and vertical element retaining wall design eliminated the need for two rows of soil nails, saving the owner more than $100,000.
• The launchable stone toe design allowed the contractor to avoid excavation below water level, reducing the project’s environmental impact.
• The retaining wall’s exterior shotcrete was stained and carved to resemble the dolomitic bedrock native to the region, allowing the wall to blend in with the surrounding environment and minimizing the project’s impact on this important tourist and fishing destination.
• GeoEngineers’ staff was on site for the duration of the project, quickly addressing questions or concerns and helping reduce construction delays.

Facts

• The project site was surrounded by a Blue Ribbon fishing area.
• The USACE required the retaining structure withstand a 100-year storm flood elevation and blend with the surrounding environment.
• Lake levels fluctuated daily by 10-15 feet, sometimes hampering construction.
• GeoEngineers completed its portion of the project on budget and two weeks early.
• The project won an American Council of Engineering Companies Grand Award in the 2010 Missouri Engineering Excellence Awards competition.
• Lead GeoEngineers Office: Springfield, MO

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